I can just about guarantee you that your stainless rifle will acquire permanent gray stains if you lean it against an aluminum rail or gunwale in an open skiff for an hour or two while underway. You can call it by whatever name you want, but the fact is that the surface of the stainless will be chemically altered.
Yes, indeed. As I said earlier, the stainless typically used, 416 or something like it, isn't very resistant to salt spray, and so will tend to rust if not protected in some way. Nothing to do with galvanic corrosion about it, and as you say, it will happen to any stainless rifle exposed to such conditions, whose surfaces are not protected in some way.
Just in passing, nothing to do with your aluminium gunwale either. If there was a galvanic cell created between the two it'd be the aluminium which is corroding, not the steel. The aluminium would form a sacrificial anode, protecting the steel from corrosion
I would certainly think the problem might be worse if less noble metals were involved as could be the case in many trigger parts.
Not impossible, but there again if you are getting staining or rust on the stainless parts of the rifle that would be mute evidence against that theory. More than likely though it is more a case of these parts being out of mind and out of sight, so if water gets in it has more of a chance to simply attack any unprotected surfaces before you notice it.
FWIW I've seen some major pitting under the wood on rifles made entirely of carbon steel, like Mauser 98s, where quite clearly there's no difference in galvanic potential. Just maintenance inadequate to the service conditions.
